An angle grinder with metal cutting disk (or abrasive disk) will remove a U-lock in about 30 seconds. It is shocking how easy it is to cut through the shank. The one consolation is that it does make a lot of noise and a lot of sparks (and requires either a power cord or an expensive battery grinder), otherwise more thieves would be using them. If you do not have access to an angle grinder (and don't want to stop by your local bike shop, auto mechanic, or police station, all of whom will undoubtedly have one), then a hacksaw with a new blade (or two) will eventually cut through, although it may take up to half an hour of vigorous sawing.
You should be able to bring over most of your previous components. However, be aware that bottom bracket sizes/types have changed considerably, the caliper distance (reach) and mounting style of brakes have changed, and there are peculiarities to seat tube and front forks based on country and year. Thus, unless you are replacing an early 1990s Italian road frame with an early 1990s Italian road frame (for example), you may find limited component capability. Pedals for almost all recent bikes (except some cheap BSOs and children's bikes) are standardized thread so those will come over fine.
Yes, dented frames are dangerous -- especially aluminium and carbon fiber as their failure modes can be quite dramatic. Steel frames tend to fail more slowly (and can be re-welded).
tl;dr: Things that you can bring over (in general): Handlebars, brifters, seat, pedals, crankset, seat, front and rear derailleurs (with caveats).
Things you might not be able to bring over (unless your bike is the same era/style): bottom bracket, steerer/stem, seat stem, brakes, forks.